This invention relates to locking the output phase of a magnetron directional amplifier to the phase of the drive signal in such a manner that high gain is achieved over a wide range of frequencies of the drive signal and temporal change in parameters that determine free running frequency of the magnetron.
In a prior art magnetron directional amplifier, shown in FIG. 1, phase locking of a magnetron is obtained by operating the magnetron 10 in combination with a passive directional device 11, such as a three-port circulator, having one port connected to a load 12 and the remaining port connected to a signal source 13. The injected drive signal for the signal source appears to the magnetron as a reflected component of the magnetron power output which acts to pull the operating frequency of the magnetron to that of the injected drive signal. A deficiency of this arrangement is that the ratio of the load power to the signal source power (the "gain") is low and is normally limited by practical considerations even if the amplifier is used at only a single frequency (no modulation). One of these considerations is that the higher the gain, the more closely the magnetron anode current level must be controlled in order to maintain phase lock. A further limitation to the use of this prior art arrangement is that the phase shift through the device is very sensitive to any difference between the operating frequency of the magnetron as a free running oscillator and the frequency of the signal source. For example, if the temperature of the anode block changes and therefore the natural frequency of the tube changes, a significant change in output phase shift from that of the input drive will occur. The sensitivity of the phase change to a number of external influences severely restricts the usefulness of the magnetron in phased arrays without additional phase shift compensation, usually provided by a phase comparator in the output and a phase shifting device connected between the signal source and the three-port circulator. The lack of high gain in the magnetron requires that the phase shifting device operate at a higher power level than that at which electronic phase shifters normally operate.
A further problem inherent in magnetron operation is the extreme sensitivity of its current flow (and therefore its output power) to a change in the voltage applied to the magnetron thereby making it necessary to use an expensive regulated power supply. Because of this same sensitivity, the phase shift through the tube is highly sensitive to any voltage ripple on the power supply.